1. Field of the Invention
The invention relates in general to ablation and electrophysiologic diagnostic and therapeutic procedures, and in particular to systems and methods for guiding and providing visualization during such procedures.
2. Related Art
Atrial fibrillation and ventricular tachyarrhythmias occurring in patients with structurally abnormal hearts are of great concern in contemporary cardiology. They represent the most frequently encountered tachycardias, account for the most morbidity and mortality, and, despite much progress, remain therapeutic challenges.
Atrial fibrillation affects a larger population than ventricular tachyarrhythmias, with a prevalence of approximately 0.5% in patients 50–59 years old, incrementing to 8.8% in patents in their 80's. Framingham data indicate that the age-adjusted prevalence has increased substantially over the last 30 years, with over-2 million people in the United States affected. Atrial fibrillation usually accompanies disorders such as coronary heart disease, cardiomyopathies, and the postoperative state, but occurs in the absence of any recognized abnormality in 10% of cases. Although it may not carry the inherent lethality of a ventricular tachyarrhythmia, it does have a mortality twice that of control subjects. Symptoms which occur during atrial fibrillation result from the often rapid irregular heart rate and the loss of atrio-ventricular (AV) synchrony. These symptoms, side effects of drugs, and most importantly, thromboembolic complications in the brain (leading to approximately 75,000 strokes per year), make atrial fibrillation a formidable challenge.
Two strategies have been used for medically managing patients with atrial fibrillations. The first involves rate control and anticoagulation, and the second involves attempts to restore and maintain sinus rhythm. The optimal approach is uncertain. In the majority of patients, attempts are made to restore sinus rhythm with electrical or pharmacologic cardioversion. Current data suggest anticoagulation is needed for 3 to 4 weeks prior to and 2 to 4 weeks following cardioversion to prevent embolization associated with the cardioversion. It remains controversial whether chronic antiarrhythmic therapy should be used once sinus rhythm is restored. Overall, pharmacologic, therapy is successful in maintaining sinus rhythm in 30 to 50% of patients over one to two years of follow-up. A major disadvantage of antiarrhythmic therapy is the induction of sustained, and sometimes lethal, arrhythmias (proarrhythmia) in up to 10% of patients.
If sinus rhythm cannot be maintained, several approaches are used to control the ventricular response to atrial fibrillation. Pharmacologic agents which slow conduction through the AV node are first tried. When pharmacologic approaches to rate control fail, or result in significant side effects, ablation of the AV node, and placement of a permanent pacemaker is sometimes considered. The substantial incidence of thromboembolic strokes makes chronic anticoagulation important, but bleeding complications are not unusual, and anticoagulation cannot be used in all patients. Medical management of atrial fibrillation, therefore, is inadequate.
In addition to medical management approaches, surgical therapy of atrial fibrillation has also been performed. The surgical-maze procedure, developed by Cox, is an approach for suppressing atrial fibrillation while maintaining atrial functions. This procedure involves creating multiple linear incisions in the left and night atria. These surgical incisions create lines of conduction block which compartmentalize the atrium into distinct segments that remain in communication with the sinus node. By reducing the mass of atrial tissue in each segment, a sufficient mass of atrial tissue no longer exists to sustain the multiple reentrant rotors, which are the basis for atrial fibrillation. Surgical approaches to the treatment of atrial fibrillation result in an efficacy of >95% and a low incidence of complications. Despite these encouraging results, this procedure has not gained widespread acceptance because of the long duration of recovery and risks associated with cardiac surgery.
Invasive studies of the electrical activities of the heart (electrophysiologic studies) have also been used in the diagnosis and therapy of arrhythmias, and many arrhythmias can be cured by selective destruction of critical electrical pathways with radio-frequency (RF) catheter ablation. Recently, electrophysiologists have attempted to replicate the maze procedure using radio-frequency catheter ablation, where healing destroys myocardium. The procedure is arduous, requiring general anesthesia and procedure durations often greater than 12 hours, with exposure to x-rays for over 2 hours. Some patients have sustained cerebrovascular accidents.
One of the main limitations of the procedure is the difficulty associated with creating and confirming the presence of continuous linear lesions in the atrium. If the linear lesions have gaps, then activation can pass through the gap and complete a reentrant circuit, thereby sustaining atrial fibrillation or flutter. This difficulty contributes significantly to the long procedure durations discussed above.
Creating and confirming continuous linear lesions could be facilitated by improved techniques for imaging lesions created in the atria. Such an imaging technique may allow the procedure to be based purely on anatomic findings.
The major techology for guiding placement of a catheter is x-ray fluoroscopy. For electrophsiologic studies and ablation, frame rates of 7–15/sec are generally used which allows an operator to see x-ray-derived shadows of the catheters inside the body. Since x-rays traverse the body from one side to the other, all of the structures that are traversed by the x-ray beam contribute to the image. The image, therefore is a superposition of shadows from the entire thickness of the body. Using one projection, therefore, it is only possible to know the position of the catheter perpendicular to the direction of the beam. In order to gain information about the position of the catheter parallel to the beam, it is necessary to use a second beam that is offset at some angle from the original beam, or to move the original beam to another angular position. Since x-ray shadows are the superposition of contributions from many structures, and since the discrimination of different soft tissues is not great, it is often very difficult to determine exactly where the catheter is within the heart. In addition, the boarders of the heart are generally not accurately defined, so it is generally not possible to know if the catheter has penetrated the wall of the heart.
Intracardiac ultrasound has been used to overcome deficiencies in identifying soft tissue structures. With ultrasound it is possible to determine exactly where the walls of the heart are with respect to a catheter and the ultrasound probe, but the ultrasound probe is mobile, so there can be doubt where the absolute position of the probe is with respect to the heart. Neither x-ray fluoroscopy nor intracardiac ultrasound have the ability to accurately and reproducibly identify areas of the heart that have been ablated.
A system known as “non-fluoroscopic electroanatornic mapping (Ben-haim; U.S. Pat. No. 5,391,199), was developed to allow more accurate positioning of catheters within the heart. That system uses weak magnetic fields and a calibrated magnetic field detector to track the location of a catheter in 3-space. The system can mark the position of a catheter, but the system relies on having the heart not moving with respect to a marker on the body. The system does not obviate the need for initial placement using x-ray fluoroscopy, and cannot directly image ablated tissue.
MR is a known imaging technique which uses high-strength magnetic and electric fields to image the body. A strong static magnetic field (between the magnet poles in this example) orients the magnetic moments of the hydrogen nuclei. RF time-varying magnetic field pulses change the spatial orientation of the magnetic moments of the nuclei. To exert a significant torque on the moment, the frequency of the magnetic field must be equal to the frequency of precession of the magnetic moment of the nuclei about the direction of the static magnetic field. This frequency of precession is a natural, or resonance, frequency of the system (hence Magnetic Resonance Imaging). The time-varying gradient magnetic field is used for spatial encoding of the signals from the issue. The magnitude of the gradient field is a linear function of the space coordinates in the magnet. As a result of the addition of the static and gradient magnetic fields, the total local magnetic field and, thus, the local resonance frequency, becomes a linear function of position. Thus, imaging tissues in any plane can be accomplished because the location of each volume element is known in three-dimensional space.
MRI is generally considered a safe technique, since no x-rays are used and the electromagnetic fields do not, by themselves, cause tissue damage.
While MRI may provide the visual guidance necessary for creating and confirming linear lesions, it has been assumed that electrical wires implanted in a patient can act as antennas to pick up radio-frequency energy in an MR system and conduct that energy to the patient, thereby causing tissue injury.
Magnetic resonance imaging has been used to guide procedures in which RF energy is applied to non-contractile organs such as the brain, liver and kidneys to ablate tumors. However, these systems are not suitable for use in the heart.
U.S. Pat. No. 5,323,778 to Kandarpa et al. discloses a method and apparatus for magnetic resonance imaging and tissue heating. There is no provision in the disclosed probe for measuring electrical signals; and, it is unclear how much resolution the probe provides.